The present invention relates to telemetered security systems and, in particular, to a crystal controlled transmitter for transmitting binary, pulse position encoded security data from distributed locations about secured premises to a system controller or central processing unit (CPU).
In partial response to changing emission standards set by cognizant regulatory authorities, a fifth overtone crystal oscillator and transmitter was previously developed for the present assignee for use in communicating security alarm data. This transmitter is described in U.S. Pat. No. 4,864,636, entitled Crystal Controlled Transmitter. The present invention comprises an improvement to that transmitter and, particularly, a lower cost oscillator circuit having improved frequency stability and output power, with fewer reactive components.
Although the oscillator circuit of the foregoing transmitter has been successfully used for some time, a frequently recurring difficulty arises from an inability to obtain uniform crystals from vendors which exhibit relatively low excess series resistance (ESR) values. That is, for the circuit to operate at the preferred frequency of 319.5 MHz, preferably an ESR of less than 30-40 OHMS is desired. Unfortunately, such values are difficult to obtain from crystal vendors.
Crystals which operate outside of these parameters either cause the oscillator stage to not work at all or to oscillate at frequencies other than the desired frequency. The use of reactive components within the collector path of the oscillator path have also demonstrated a sensitivity of the circuitry to the physical arrangement of the components on a printed circuit board and within the transmitter package. Accordingly, efforts have been made to develop an improved circuit which is less sensitive to variations in the ESR value of the crystal and to the physical arrangement of the circuitry. Additional efforts have been made to reduce the component count and number of reactive components and to improve the temperature sensitivity and output power capabilities of the oscillator. Such goals are attained with the circuitry of the present invention.